Neutrophil emigration during inflammation is a highly regulated process and is critical for host defense against invading organisms. However, excessive accumulation and activation of neutrophils can lead to damaging effects. Neutrophil serine proteases [neutrophil elastase (NE), cathepsin G (CG), proteinase 3 (PR3) are expressed specifically in mature neutrophils. We have recently created a mouse with a null mutation in the lysosomal cysteine protease dipeptidyl peptidase I (DPPI). We showed that DPPI is required for the processing and activation of many serine proteases including NE, CG, and PR3. Furthermore, our preliminary results indicate that the DPPI- deficient and NE x CG-deficient mice are protected against acute arthritis induced by passive transfer of monoclonal antibodies to type II Collagen. Specifically, there is no accumulation of neutrophils in the joints of protease-deficient mice. These results support the hypothesis that DPPI (probably through the action of NE and CG) plays a non-redundant role in neutrophil emigration and/or neutrophil recruitment at specific inflammatory sites. Thus, we propose the following specific aims: 1. We will define the role(s) of DPPI and serine proteases in neutrophil migration. Neutrophil migration requires the activation and interaction of several adhesion molecules. In this specific aim, we will examine the expression of adhesion molecules L-selectin and beta2 integrins upon activation with LPS. We will also examine the requirement for DPPI and serine proteases in neutrophil adhesion and transmigration in vitro. 2. We will define the in vivo role(s) of DPPI and serine proteases in the model of acute arthritis. Our preliminary results suggest that there is either a primary defect in neutrophil emigration or a secondary defect in neutrophil activation and recruitment at sites of inflammation. In this aim, we will characterize the direct response of DPPI-deficient mice to intra-articular IL-8. We will also establish chimeric mice by bone marrow transplantation to determine whether accumulation and activation of wild type neutrophils at sites of inflammation will be sufficient to recruit DPPI-deficient neutrophils. 3. We will explore the role(s) of neutrophil-derived serine proteases in other models of acute and chronic inflammation. In this aim, we will determine whether the defect in neutrophil emigration/recruitment is generalized to other organs, such as skin. In addition, we will also study two additional arthritis models, one of septic arthritis and the other involving chronic inflammation.